A great many vehicles, such as snowmobiles, have a variable speed belt transmission driving system (sometimes referred to as torque converters). In such a system, there is a drive or driving clutch with a moveable conical faced sheave and a fixed conical faced sheave, a driven clutch with a moveable conical faced sheave and a fixed conical faced sheave, a transmission belt extending between each pair of sheaves coupling the driving and driven clutches, a speed responsive or displacement means (hereinafter referred to as "speed responsive means" but that term also represents "displacement means" and the two terms can be interchanged) such as a plurality of like cams, flyweights or other means which are operatively associated with the moveable sheave of the driving clutch and a biasing or resistance means in the driving and driven clutch to oppose the displacement of the moveable sheave by the speed responsive means.
The purpose of the driving clutch is to control the speed of the engine in all gear ratios as the transmission changes gears. There is a biasing or resistance means in the driving clutch that works against the speed responsive means associated with the moveable sheave. The biasing means and speed responsive means are matched to assure a reasonable initial engagement speed to assure that the vehicle will have enough power to move. The drive clutch is the primary mechanism to determine the upshift of the clutch system and as such should be used to determine engine rpm. The driving clutch can be adjusted to achieve a predetermined or desired engine speed by modifying the biasing or resistance means (see commonly owned co-pending application, U.S. patent application Ser. No. 08/294,043 filed Aug. 24, 1994, now U.S. Pat. No. 5,460,575 issued Oct. 24, 1995, which is incorporated herein by reference in its entirety to which this application is a continuation-in-part continuing application) or, by modifying the speed responsive means in accordance with the present invention.
The purpose of the driven clutch is to control the backshift and to provide enough side pressure on the transmission belt to allow power to be transmitted from the driving clutch to the driven clutch without the belt slipping. The side pressure on the belt has a lot to do with power loss and backshifting. The higher the belt pressure, the quicker the backshifting but the higher the power loss, also. Therefore, conventionally the driven clutch biasing means is selected to be a compromise between quick backshifting and low power loss. High belt side pressure also results in increased belt wear and shorter belt life.
In operation at low rotational speeds of the driving clutch, the fixed and moveable sheaves of the driving clutch are forced apart by a biasing or resistance means (e.g., compression spring) parallel to the centerline of a drive shaft running between the fixed and axially moveable sheaves, thus enabling the transmission belt to ride near the bottom of the driving clutch thereby creating a small diameter drive wheel. Correspondingly, the sheaves of the driven clutch are forced close together by a biasing means parallel to the centerline of a drive shaft running between the fixed and moveable sheaves, thus causing the transmission belt to operate near the outer edge of the conical faces of the driven clutch sheaves thereby creating a large diameter driven wheel. Therefore, at low speeds a small diameter drive wheel clutch is coupled by the transmission belt to a large diameter driven wheel. This is, in effect, a low gear since it requires many turns of the drive wheel to cause one rotation of the driven wheel.
As the rotational speed of the driving clutch increases in response to increased engine speed, speed responsive means (i.e., cams or flyweights) operatively associated with the moveable sheave of the driving clutch, and opposed by the biasing or resistance means located on or parallel to the centerline of the driving clutch as well as side pressure on the transmission belt caused by the biasing means in the driven clutch, force the moveable sheave of the driving clutch closer to the fixed sheave thereby causing the transmission belt to move outward on the conical radius of the drive wheel so as to operate at a greater distance from the center of the driving clutch. The forces on the transmission belt which cause it to move upward along the conical radius of the drive wheel also cause it to move inwardly against the side pressure along the radius of the driven wheel thus forcing the moveable sheave of the driven clutch away from its associated fixed sheave. These sheaves are spread apart in relation to the driven helix and spring in the driven clutch. The steeper the angle on the helix, the faster the sheaves will spread. The spring counteracts this force to keep the sheaves from shifting too fast, as well as providing a back shift force when the engine speed is reduced.
Thus higher rotational speeds of the driving clutch cause the transmission belt to effectively form a large diameter drive wheel and a small diameter driven wheel. This is, essentially, a high gear since it enables one complete rotation of the drive wheel to cause several complete rotations of the driven wheel. This means that such a transmission belt drive system has a built-in capability of effectively switching from a low gear to a higher gear as the rotational speed of the drive wheel is increased.
However, there are some inherent disadvantages to this type of system in which the gear ratio is automatically changed with an increase or decrease in rotational speed of the drive clutch. These disadvantages exist because the gear ratio change can occur at a time when constant speed is desired, such as when travelling down a road or trail, or the gear ratio can remain fixed at a time when it should be changing in response to an increased load, like a hill or a turn.
Consider, for example, a snowmobile which utilizes the transmission belt drive system. Normally, a low gear is needed to get the machine moving but after it has commenced moving and the throttle is advanced, the gear ratio begins to change in order to place the vehicle in a higher gear. This is normal operation and no problem occurs.
When it is desired to travel at a constant speed the transmission will operate in a higher gear ratio than is needed if the clutch has been tuned to operate at the top of the power band thereby forcing the operator to drive in a higher gear than is necessary for the given conditions. This results in excessive power output and thus poor fuel economy.
Further, assume it is desired to climb a hill or in some other manner the load placed upon the vehicle is increased. If the vehicle is travelling at a high rate of speed the driving clutch rotational speed is high and the machine is in a high gear ratio. However, when a vehicle is attempting to climb a hill or move a heavy load under increasing load conditions, it needs a lower gear ratio, therefore such a higher gear ratio is an undesirable situation. That is, at this time greater torque is needed at the driven wheel, not greater speed. In order to achieve greater torque, the transmission must backshift. Failure to do so results in power fuel efficiency and undesirable air pollution.
Therefore, a particular disadvantage of this type of known system is that the vehicle is slow to backshift, (i.e., downshift) in response to this need for greater torque. The reason being that the speed responsive means (i.e., flyweights) that push the sheaves of the driving clutch together against the compression spring in the driving clutch and the compression spring that initially pushes the sheaves of the driven clutch together is a fixed combination that is selected to compromise between a combination that upshifts quickly and a combination that downshifts quickly. If the flyweights (i.e., speed responsive means) are light, the transmission will upshift slowly but downshift quickly. If the flyweights are heavy, the transmission will upshift quickly but downshift slowly.
There is a long-felt need for a device which will overcome these problems and allow the driving system to upshift quickly when low drag is encountered and downshift quickly when more torque is needed in addition to allowing the engine to be operated at either its most fuel efficient speed setting or its most powerful speed setting.